The present invention relates generally to valve actuation in internal combustion engines that include compression release-type engine retarders. In particular, it relates to methods and apparatus for controlling valve lift and duration for compression release valve events and main exhaust valve events.
Engine retarders or brakes of the compression release-type are well-known in the art. Engine retarders are designed to convert, at least temporarily, an internal combustion engine of compression-ignition type into an air compressor. In doing so, the engine develops retarding horsepower to help slow the vehicle down. This can provide the operator increased control over the vehicle and substantially reduce wear on the service brakes of the vehicle. A properly designed and adjusted compression release-type engine retarder can develop retarding horsepower that is a substantial portion of the operating horsepower developed by the engine in positive power.
Functionally, compression release-type retarders supplement the braking capacity of the primary vehicle wheel braking system. In so doing, it extends substantially the life of the primary (or wheel) braking system of the vehicle. The basic design for a compression release engine retarding system of the type involved with this invention is disclosed in Cummins, U.S. Patent No. 3,220,392 (November 1965) for a Vehicle Engine Braking And Fuel Control System.
The compression release-type engine retarder disclosed in the Cummins ""392 patent employs a hydraulic system or linkage. The hydraulic linkage of a typical compression release-type engine retarder may be linked to the valve train of the engine. When the engine is under positive power, the hydraulic linkage may be disabled from providing valve actuation. When compression release-type retarding is desired, the hydraulic linkage is enabled such that valve actuation is provided by the hydraulic linkage responsive to an input from the valve train.
Among the hydraulic linkages that have been employed to control valve actuation (both in braking and positive power), are so-called xe2x80x9clost-motionxe2x80x9d systems. Lost-motion, per se, is not new. It has been known that lost-motion systems are useful for variable valve control for internal combustion engines for decades. In general, lost-motion systems work by modifying the hydraulic or mechanical circuit connecting the actuator (typically the cam shaft) and the valve stem to change the length of that circuit and lose a portion or all of the cam actuated motion that would otherwise be delivered to the valve stem to produce a valve opening event. In this way lost-motion systems may be used to vary valve event timing, duration, and the valve lift.
Compression release-type engine retarders may employ a lost motion system in which a lash piston is included in the valve train (e.g. a linkage of a push tube, cam, and/or rocker arm) of the engine. When the retarder is engaged, the lash piston is hydraulically extended to cause the exhaust valve of the internal combustion engine to open at a point near the end of a piston""s compression stroke. In doing so, the work that is done in compressing the intake air cannot be recovered during the subsequent expansion (or power) stroke of the engine. Instead, it is dissipated through the exhaust and radiator systems of the engine. By dissipating energy developed from the work done in compressing the cylinder gases, the compression release-type retarder dissipates the kinetic energy of the vehicle, which may be used to slow the vehicle down.
Regardless of the specific actuation means chosen, inherent limits were imposed on operation of the compression release-type retarder based on engine parameters. One such engine parameter is the physical relationship of an engine cylinder valve used for compression release braking and the piston in the same cylinder. If the extension of the valve into the cylinder was unconstrained during compression release braking, the valve could extend so far down into the cylinder that it impacts with the piston in the cylinder.
There may be a significant risk of valve-to-piston contact when a unitary cam lobe is used to impart the valve motion for both the compression release valve event and the main exhaust valve event. Use of a unitary cam lobe for both events means that the relatively large main exhaust lobe motion will be imparted to the hydraulic linkage, or more particularly to the slave piston. Because there is typically little or no lash between the lash piston and the exhaust valve during engine braking, input of the main exhaust event motion to the lash piston may produce a greater than desired main exhaust event. A means for limiting the downward stroke of an exhaust valve for its main exhaust event during engine braking is needed.
Some systems do not use a unitary cam lobe for both the compression release valve event and the main exhaust valve event. These systems may operate using a dedicated braking cam lobe to drive a dedicated braking rocker arm, and a dedicated main exhaust cam lobe to drive a dedicated main exhaust rocker arm. The braking and main exhaust rocker arms may actuate different or the same exhaust valves using one or more bridges or similar arrangements to convey the rocker arm motions to the selected exhaust valves. Although these xe2x80x9cdedicatedxe2x80x9d systems do not run the same risks of valve-to-piston contact as the xe2x80x9cunitary camxe2x80x9d systems, they may also benefit from inclusion of a means to limit the downward stroke of the exhaust valves.
One way of limiting the downward stroke of an exhaust valve used for compression release valve events and/or main exhaust valve events is to limit the extension of the hydraulic lash piston that is responsible for pushing the valve into the cylinder during compression release braking. A device that may be used to limit piston extension or motion is disclosed in Cavanagh, U.S. Pat. No. 4,399,787 (Aug. 23, 1983) for an Engine Retarder Hydraulic Reset Mechanism, which is incorporated herein by reference. Another device that may be used to limit piston motion is disclosed in Hu, U.S. Pat. No. 5,201,290 (Apr. 13, 1993) for a Compression Relief Engine Retarder Clip Valve, which is also incorporated herein by reference. Both of these (reset valves and clip valves) may comprise means for blocking a passage in a lash piston during the downward movement of the lash piston (such as the passage 344 of the slave piston 340 of FIG. 6). After the lash piston reaches a threshold downward displacement, the reset valve or clip valve may unblock the passage through it and allow the oil displacing it to drain there through, causing the lash piston to return to its upper position under the influence of a return spring.
A reset valve, such as the one disclosed in Cavanagh, may be provided as part of a lash adjuster or a lash piston. A reset valve may comprise a hydraulically actuated means for unblocking a passage through the lash piston to limit its displacement. In Cavanagh, compression release retarding is carried out by opening one of two valves connected by a crosshead member or bridge. A purpose of the reset valve used in Cavanagh is to reseat the exhaust valve used for the compression release event before a subsequent main exhaust valve event so that the rocker arm will not push down on an unbalanced crosshead during the main exhaust event and transmit a bending force to the crosshead guide pin or to the non-braking valve stem.
A clip valve, such as the one disclosed in Hu, may comprise a mechanically actuated means for unblocking the passage through a hydraulically extendable piston to limit its extension.
As evident from the foregoing, compression release retarding systems have historically been implemented as bolt-on systems added to an existing engine as an optional or after-market item. As the market for compression release-type engine retarders has developed and matured, the direction of technological development has moved away from bolt-on systems towards compact, cost-efficient integrated engine braking systems. More and more engine manufacturers have expressed an interest in incorporating or integrating the engine brake components into their fundamental engine designs in order to achieve their cost and performance goals. It is believed that incorporation of the engine brake into the engine will ultimately provide the needed cost, weight, performance, and efficiency benefits.
One method of engine brake integration is disclosed in Cartledge, U.S. Pat. No. 3,809,033 (May 7, 1974) for a Rocker Arm Engine Brake System. With reference to FIGS. 6-8 of Cartledge, a rocker arm 16 incorporates a lash piston 31 that may be hydraulically extended from the rocker arm for braking operation. The rocker arm transfers braking motion from a cam (not shown) to an exhaust valve 15. The lash piston 31 takes up the lash between the rocker arm 16 and its associated exhaust valve during engine braking. The elimination of this lash during braking allows a small braking lobe on the exhaust cam to produce a compression release opening of the exhaust valve near the top of the piston compression stroke.
A more recent development of the rocker arm brake is disclosed in McCarthy, U.S. Pat. No. 5,975,251 (Nov. 2, 1999) for a Rocker Brake Assembly With Hydraulic Lock, which is incorporated herein by reference. With reference to FIG. 1 of McCarthy, a rocker arm assembly 10 having a brake rocker arm 100 mounted on a rocker shaft 200 is shown. The brake rocker arm 100 pivots about the rocker shaft 200 and includes a first end 110 and a second end 120. The first end 110 of the brake rocker arm 100 includes a brake cam lobe follower 111. The brake cam lobe follower 111 may include a roller 112 that is in contact with a brake cam lobe, not shown. The second end 120 of the brake rocker arm 100 includes an actuator assembly 121. The actuator assembly 121 is spaced from the crosshead of an exhaust rocker arm, not shown. When activated, the brake rocker arm 100 and the actuator assembly 121 contact the crosshead pin, not shown, of the crosshead to open the at least one exhaust valve to perform a braking operation. The brake rocker arm 100 also includes a fluid passageway 130 that extends from the actuator assembly 121. Hydraulic fluid from a passageway 210 in the shaft 200 may be supplied to the fluid passageway 130 to operate the actuator assembly 121.
Furthermore, both current and expected environmental restrictions have forced engine manufacturers to explore a variety of new ways to improve the efficiency of their engines. These changes have forced a number of engine modifications. Engines have become smaller and more fuel efficient, increasing the need for weight saving integration of engine brakes. Yet, the demands on retarder performance have often increased, requiring the compression release-type engine retarder to generate greater amounts of retarding horsepower under more limiting conditions.
In view of the foregoing, there is a need for an integrated engine braking system and method of operation therefor, that includes a lash piston that may be hydraulically reset and/or clipped. In particular, there is a need for an engine braking system having a lash piston and a means for resetting or clipping the lash piston integrated into a rocker arm assembly.
It is therefore an object of the present invention to provide an actuation means for engine braking that optimizes engine retarding performance.
It is another object of the present invention to provide a system and method for avoiding valve-to-piston contact during a main exhaust valve event.
It is a further object of the present invention to provide a system and method for limiting the stroke of a lash piston during an engine valve opening event.
It is yet another object of the present invention to provide a system and method for resetting a lash piston following an engine valve opening event.
It is still another object of the present invention to provide a system and method for clipping the motion of a lash piston during an engine valve opening event.
It is still a further object of the present invention to provide a system and method of engine braking that is integrated into the rocker arm/shaft assembly.
Additional objects and advantages of the invention are set forth, in part, in the description which follows, and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.
In response to this challenge, Applicants have developed an innovative and reliable engine braking system, for providing a compression release valve event in an internal combustion engine, comprising: a rocker arm shaft; a rocker arm having a central bore adapted to receive the rocker arm shaft; means for pivoting the rocker arm on the rocker arm shaft to provide a compression release valve event; an hydraulically extendable lash piston disposed in a piston bore in the rocker arm, said lash piston being adapted to open an engine valve for the compression release event; means for providing hydraulic fluid to the piston bore; an hydraulic relief port provided on the rocker arm, said relief port having hydraulic communication with the piston bore; and means for selectively unblocking the relief port responsive to pivoting of the rocker arm.
Applicants have also developed an engine braking system, for providing a compression release valve event in an internal combustion engine, comprising: a rocker arm shaft; an hydraulic relief passage formed in the rocker arm shaft, said relief passage communicating with an outer surface of the rocker arm shaft; a rocker arm having a central bore adapted to receive the rocker arm shaft; means for pivoting the rocker arm on the rocker arm shaft to provide a compression release valve event; an expandable hydraulic tappet disposed in a piston bore in the rocker arm, said tappet being adapted to open an engine valve for the compression release event; means for providing hydraulic fluid to the tappet; and means for providing selective hydraulic communication between the relief passage and the tappet responsive to pivoting of the rocker arm.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed. The accompanying drawings, which are incorporated herein by reference, and which constitute a part of this specification, illustrate certain embodiments of the invention and, together with the detailed description, serve to explain the principles of the present invention.